1. Field of the Invention
This invention relates to a semiconductor light emitting device or other like devices and, more particularly, to a semiconductor light emitting device comprising II-VI compound semiconductors.
2. Related Arts
Recently, II-VI compound semiconductors are used as materials of light emitting devices such as light emitting diodes, semiconductor lasers, and so on. They are fabricated by epitaxial growth of one or more II-VI semiconductors on a (100)-oriented semiconductor substrate by, for example, molecular beam epitaxy (MBE).
Examples of short-wavelength light sources (mainly, blue light sources), for use as light sources for high-density optical recording and displays, include light emitting devices in which a II-VI compound semiconductor such as ZnCdSe, or the like, is stacked by crystal growth by MBE. These light emitting devices have attained continuous oscillation at room temperatures.
Upon making the epitaxially grown layer on the substrate, defects such as stacking faults occur along the interface between the substrate and the epitaxially grown layer. In order to restrict growth of such defects, some attempts have been made to improve the pre-treatment of the substrate to be performed before crystal growth or to improve the method of growth at the beginning of the epitaxial growth.
However, lifetimes of continuous oscillation of these light emitting devices at the room temperature are still as short as only several seconds, far from practically acceptable levels.
One of the reasons is insufficient crystalline qualities of active layers for light emission, guide layers and cladding layers.
It is known through TEM observation that II-VI compound semiconductor layers made by epitaxial growth ordinarily include about 10.sup.6 .about.10.sup.7 cm.sup.-2 defects.
Moreover, stacking faults often occur at the interface between the base body and II-VI compound semiconductors stacked by epitaxial growth.
If the guide, or active, layer is made on the epitaxially grown layer including such defects, then its crystal growth progresses while trailing the defects, and does not result in a good quality. If such defects remain, they move or multiply during laser oscillation due to the process of recombination of excessive carriers in the crystal, which causes deterioration of the crystalline quality at a great speed and deterioration of the oscillating characteristic of the light emitting device.
The defects also cause problems regarding the optical characteristics of the grown crystal, such as light emission from a deep level, non-radiative center, or the like. They further give a bad influence to electric characteristics such as compensation of impurities. These factors invite a decrease in light emitting characteristics of light emitting devices.
Moreover, existing II-VI compound optical semiconductor devices use a ZnCdSe single-quantum well layer as their active layer. This layer, however, is lattice-mismatched with a GaAs substrate, and a compressive stress is applied to the active layer. This stress is a factor that promotes multiplication of defects during electrical energization under the existence of misfit dislocations in the active layer. Due to the existence of a number of defects in the active layer, in addition to the structure liable to promote the motion of dislocation, semiconductor lasers so far available have only short lifetimes.